Control unit with three parallel branches

ABSTRACT

This controller, between a fixed base ( 1 ) and a control handle ( 4 ), comprises three similar arms ( 2 ) of which each comprises three links ( 5,6,7 ) jointed between the base and the handle platform by a pivot ( 8 ), two rotation joints ( 9  and  10 ) and a ball-joint ( 11 ). This controller with six degrees of freedom includes force feedback motors at the first and second joints ( 8  and  9 ) of which the first is fixed to the base ( 1 ) and the second to the first link ( 5 ), so that the first is immobile and the other has scarce movement; the third joint ( 10 ) is free. The resulting structure is simple and light not requiring any parallelogram, and kinematic singularities are greatly reduced.

The invention concerns a controller with three parallel branches.

Controllers are intended for varied procedures such as remote handling,remote surgery, games, or the controlling of other machines by recordingand communicating movements which the operator causes them to undergo.Their structures comprise a variable number of degrees of freedom withwhich to command en equal number of operations.

Different types of structures for controllers are known, but the mostwidespread comprise a succession of mutually jointed links between agenerally fixed base and a handle held by the operator. Thesesuccessions of links are called arms. Often single-arm types are foundand the controller is then a jointed arm or “serial arm” which, toobtain a great number of degrees of freedom, requires either a highnumber of onboard motors at the different joints or complextransmissions if the motors are placed close to the base. In all cases,mounting is necessarily complex. Controllers with two or three arms arealso known mounted in parallel between the base and the handle withwhich it is possible to obtain a large number of degrees of freedomwhile maintaining simple arm structure. The arms then converge towards aplatform carrying the handle and are connected to it by double joints(cardan or universal joints) or triple joints (ball joints).

Several pitfalls must be avoided by the designer of controllers of thistype: it is indicated that configurations should be avoided which cannotbe attained or which can only be attained via an indeterminate movementunable to generate a proper command; the configurations of the firsttype often relate to collisions between arm links and those of thesecond type to kinematic singularities derived for example from couplingbetween joint movements. It is further preferable to limit themechanical complexity of the structure. Finally, it is advantageous thatthe so-called force feedback motors, which are needed to retain thejoints at their end-point position to which they were brought by theoperator and to oppose resistance to movement for more comfortablecontrolling, should be mounted on the base rather than on the mobilearms since their volume lends to more probable collisions and theirweight has to be supported by arms of sufficient section to carry suchweight and are therefore heavier. Also, arm movements may cause avariation in the bending forces exerted by the motors on account oftheir overhang and hence in the equilibrium conditions of thecontroller.

It is evident that these conditions cannot all be met simultaneously andthat no ideal structure exists for a controller comprising for examplesix degrees of freedom and three parallel arms. A controller of thistype exists whose six motors associated with the six degrees of freedomare all mounted on the base, but the three arms are connected to thebase via double joints which, in addition to greater complexity indesign and adjustment than single joints, require couplings of movementwhich are detrimental to the efficacy of force transmission. Onebibliographical source is the article “Kinematic analysis of a novel6-DOF parallel manipulator” by Cleary and Brooks, IEEE; 1050-4729, 1993,pp. 708 to 713.

In several other controllers some links are split into a parallelogramor pantograph. Instead of having to arrange motors on the two joints atthe ends of these links, the motors may be placed on two lower apexes ofthe parallelogram or pantograph to exert a smaller overhang, but thesemotors are therefore all mobile and dividing the link is not excellent.One example is the controller developed by Iwata called “Haptic Master”described on the Tsukuba internet site(http:/:intron/kz.tsukuba.ac.jp/HM/txt.html); and others are given inthe article ‘A 6-DOF force-reflecting hand controller using the fivebarparallel mechanism” by Woo, Jin and Kwon, Proceedings of the IEEE,International Conference on Robotics & Automation, Louvain, Belgium, May1998, pp. 1597-1601.

The controller of the invention is characterized by great simplicity ofstructure combined with a small number of mobile motors. Also the mobilemotors are not positioned unfavourably, i.e. they are not likely toproduce a substantial, variable overhang or to enter easily intocollision with other controller parts.

To summarize, the controller of the invention which comprises threeparallel arms connecting a base to a platform carrying a handle (or anyother grasping means) is characterized in that the arms are made up ofthree links, of which a first link joined to the base by a first jointwhich is a pivot joint of the first link about itself, a second linkjoined to the first link by a second joint which is a rotation joint tomodify an angle between the first link and the second link, a third linkjoined to the platform by a ball-joint and to the second link by a thirdjoint which is a rotation joint to modify an angle between the secondlink and the third link, and in that the arms only comprise two forcefeedback motors of which a first motor fixed to the base and measuringpivot movements of the first link and a second motor positioned on thesecond joint and measuring rotations between the first link and thesecond link.

Details of the advantages obtained are described below. It can be addedhowever that kinematic singularities are considerably reduced byimplanting the first links on the base in diverging directions and,preferably, at an incline of around 40° to the vertical (or moregenerally on a normal to the plane passing through the three implantpoints of the first links). This value of 40° is only approximate, thereis no clear limit to angle of incline that is recommended, and it wassimply found that singularities were substantially less probable ataround this value.

The invention will now be described with reference to the figures, ofwhich FIG. 1 is a general view of the invention and FIGS. 2 and 3illustrate a favourite embodiment with details of the force feedbackmotors and link joints.

With reference to FIG. 1. A base carries reference 1 and the controllerchiefly comprises three similar arms, all referenced 2 and made up ofthree links jointed between base 1 and platform 3 to which a handle 4 isfixed held by the operator; platform 3 may be flat as is frequent or, ashere, ball-shaped. Each of arms 2 comprises a first link 5 jointed withbase 1, a third link 7 jointed with platform 3, and a second link 6jointed with the two previous links. Arms 2 further comprise a firstjoint 8 between base 1 and the first link 5, a second joint 9 betweenthe first and second links 5 and 6, a third joint 10 between the secondand third links 6 and 7, and a ball-joint 11 between the third link 7and the platform 3. The first joint 8 is a pivot joint, i.e. its axis isco-linear to the first link 5, which may therefore rotate about itselfon base 1; the second and third joints 9 and 10 are rotation joints,which enable changing of the angles between the links leading to them,namely the second link 6 and respectively the first link 5 and the thirdlink 7. In this configuration, the axes of these rotation joints aretherefore perpendicular to the links which they connect and, inaddition, they are parallel to one another. In other embodiments this isnot necessarily the case. It is seen that the platform 3 may be movedalong the usual three degrees of freedom for translation in space and inrotation about three different axes, by movements reflected by arms 2and non-resisted by these arms. The six degrees of freedom of thecontroller are therefore obtained.

With reference now to FIGS. 2 and 3, in which for reasons of clarity theplatform and the handle are not shown, which illustrate a slightlydifferent embodiment in which the first links 5 are not parallel to oneanother and vertical to a planar, horizontal base 1, but are inclined indiverging directions forming an angle α of around 40° relative to thenormal of base 1. This arrangement makes it possible to distanceoutwardly the third links 7 which are close to one another in thepreceding embodiment, and hence to reduce the risk of collision betweenthese third links 7 and also to move kinematic singularities outside theeffective workspace. Base 1 then comprises inclined pedestals 12 onwhich the first links 5 are mounted via a bearing 13 which materializesthe first joint 8. The foot of the first links 5 carries a pulley 14around which a belt 15 is tensioned; the other end of the belt 15 istensioned around the shaft of a motor 16 also fixed to pedestal 12 andwhich comprises a encoder 17 measuring the rotational movements of theshaft. A device of this type forms force feedback means via which themotor 16, through its resistance, maintains link 5 at rest at theposition it has reached; but when a movement is imposed upon the firstlink 5, the encoder 17 records the same enabling control of the machine,not shown, dependent upon the controller. This motor 16 is fixed sinceit is attached to base 1, which is advantageous for the above-mentionedreasons. It will be noted that to actuate part 14, any other equivalentmeans may be used (gearing, cable, etc . . . ).

Rotation joints 9 and 10 are each materialized by a pin supported by aclevis 21 attached to one of the links connected by the joint, and theother link is fixed to pin 20. Pin 20 attached to the second link 6 inthe second joint 9 is jointed with clevis 21 attached to the first link5 by bearings 22. Clevis 21 carries a motor 23, and pin 20 carries apulley 24. A belt 25 or any other transmission means, is tensionedbetween pulley 24 and the shaft of motor 23, and an encoder 26 measuresthe shaft movements of the motor 23. This is a force feedback devicethat is similar to the previous one. Rotation movements of the secondlink 6 relative to the first link 5 are recorded by the encoder 26 andthe motor 23 supports the second link 6 at the position it has reached.

The third joint 10 is similar in structure to the second joint 9, but itis smaller since it does not include a force feedback motor: it is freeand passive just like ball-joint 11. Therefore the force feedback motor23 is not fixed to the base but only accompanies the pivot movements ofthe first link 5 and therefore makes little movement, which reducesunbalance and risks of collision it could possibly cause almost to thesame extent as if it had been fixed to the base. It will be noted thatlinks 5, 6 and 7 are simple i.e. they do no comprise any parallelogramor pantograph.

1. A controller comprising three parallel arms (2) connecting a base (1)to a platform (3) carrying grasping means such as a handle (4), the armscomprising three links (5,6,7) of which a first link (5) is joined tothe base by a first joint (8) which is a pivot joint of the first linkabout itself, a second link (6) joined to the first link (5) by a secondjoint (9) which is a rotation joint to modify an angle between the firstlink and the second link, a third link (7) joined to the platform (3) bya ball-joint (11) and to the second link (6) by a third joint (10) whichis a rotation joint to modify an angle between the second link and thethird link, wherein each of the arms only comprise two force feedbackmotors (16,23), of which a first motor (16) fixed to the base (1,12) andmeasuring pivot movements of the first link (5) and a second motor (23)positioned on the second joint (6) and measuring the rotations betweenthe first link and the second link.
 2. The controller as in claim 1,wherein the first links (5) are implanted on the base (1,12) indiverging directions.
 3. The controller as in claim 2, wherein the firstlinks (5) are implanted on the base (1,12) at an incline of 40° relativeto a direction perpendicular to the base.
 4. The controller as in claim2, wherein the first links (5) are implanted on the base (1,12) at anincline of 40° relative to a direction perpendicular to the base.
 5. Thecontroller as in claim 1, wherein the first links (5) are implanted onthe base (1,12) in diverging directions.
 6. A controller consisting of abase, a platform carrying grasping means such as a handle (4), threeparallel arms (2) connecting a base (1) to the platform (3) and sixforce feedback motors, the arms comprising three links (5,6,7) of whicha first link (5) is joined to the base by a first joint (8) which is apivot joint of the first link about itself, a second link (6) joined tothe first link (5) by a second joint (9) which is a rotation joint tomodify an angle between the first link and the second link, a third link(7) joined to the platform (3) by a ball-joint (11) and to the secondlink (6) by a third joint (10) which is a rotation joint to modify anangle between the second link and the third link, wherein each of thearms comprises two force feedback motors (16,23), of which a first motor(16) fixed to the base (1,12) and measuring pivot movements of the firstlink (5) and a second motor (23) positioned on the second joint (6) andmeasuring the rotations between the first link and the second link.